Collision avoiding system and protocol for a two path multiple access digital communications system

Abstract

In order to control the transfer of packets of information among a plurality of stations, e.g., digital computers, the instant communications system, station and protocol contemplate first and second oppositely directed signal paths. At least two stations are coupled to both the first and the second signal paths. A station includes arrangements for reading one signal from the first path and for writing another signal on the first path. The one signal is read from the first path by an arrangement which electrically precedes the arrangement for writing the other signal on the first path. A similar read arrangement is electrically preceding a write arrangement on the second path. If the station has a packet to transmit, it can overwrite a busy control field of the one signal packet on either path. Having read the one signal on the path, a logical interpretation may be made within the station as to whether the path is busy or is not busy. If the path is not busy, the packet may be written on the path by overwriting any signal thereon. If the path is busy, the station may defer the writing until the path is detected as not busy through a busy control field. The packets flow down the path to a destination station. Eventually all packets will be transmitted and the busy control field may be detected at an end station for indicating the event that all packets have been transmitted. The end station on one path acts as the head station on the other oppositely directed path. Hence, responsive to detecting that all packets have been transmitted, the head station on the other path may transmit an end cycle packet, responsive to which event a new cycle may be started by initializing each station on the one path and by permitting each initialized station to transmit.

What is claimed is: 
     
       1. A method for controlling digital signals in a communications system, said system including a plurality of stations, at least two of said stations being coupled to both a first and a second signal path, said method including the steps of writing signals on said first path and reading signals from said second path and characterized in that said signals on said first path include first control and data signals,   said signals on said second path include second control and data signals,   said first and second paths are, separate, independent, unidirectional and oppositely directed with respect to each other, and   said method further comprising the steps of   reading said first control signals from said first path, said first path reading step occurring logically before said writing on said first path,   responsive to first control signals read from said first path avoiding a collision on said first path,   writing second control signals on said second path, said second path writing step occurring logically after said reading from said second path   responsive to second control signals read from said second path, avoiding a collision on said second path.   
     
     
       2. The method defined in claim 1 wherein said respective collision avoiding step further comprises the step of overwriting a busy control field of a first packet on said respective path with a signal for indicating that a station has a second packet to transmit on said respective path.   
     
     
       3. The method defined in claim 2 wherein said respective collision avoiding step further comprises the steps of reading a busy control field of said first packet, said first packet busy control field having been read prior to its being overwritten, and   responsive to said priorly read busy control field indicating that said respective path is not busy and responsive to said station having said second packet to transmit, entering an access state and writing said second packet on said respective path, said second written packet thereby overwriting any packet on said first path.   
     
     
       4. The method defined in claim 2 wherein said respective collision avoiding step further comprises the steps of reading a busy control field of said first packet, said first packet busy control field having been read prior to its being overwritten, and   responsive to said priorly read busy control field indicating that said respectve path is busy, entering a defer state until a priorly read busy control field indicates that said respective path is not busy.   
     
     
       5. The method defined in claim 2 wherein said respective collision avoiding step further comprises the steps of: reading a busy control field of said first packet, said first packet busy control field being read prior to its being overwritten,   responsive to said priorly read busy control field indicating that said respective path is not busy and responsive to the event that no station on said respective path was unable to transmit a packet, writing an end cycle packet on said other respective path.   
     
     
       6. The method defined in claim 2 wherein said respective collision avoiding step further comprises the steps of reading said end cycle packet on said other respective path and   responsive to said end cycle packet, initiating a start cycle packet on said respective path, said start cycle packet being transmitted by a head station on said respective path and said start cycle packet including a start cycle field responsive to which a station on said respective path may be initialized to transmit a packet.   
     
     
       7. A communications system comprising a first and a second communications signal path, a plurality of stations, each of at least two of said stations being coupled to both said first and said second signal paths, a station including means for writing a first signal on said first path and means for reading a second signal from said second path and characterized in that said signals on said first path include first control and data signals,   said signals on said second path include second control and data signals,   said first and second paths are, separate independent, unidirectional and oppositely directed with respect to each other, and   said system further comprises:   means for avoiding collisions on said first communications path, said first path collision avoiding means including   means for reading a third signal from said first control signals on said first path, said first path reading means being coupled to said first path in a manner logically preceding said first path writing means,   means for avoiding collisions on said second communications path, said second path collision avoiding means including means for writing a fourth signal as one of said second control signals on said second path, said second path writing means being coupled to said second path in a manner logically succeeding said second path reading means.   
     
     
       8. The system defined in claim 7 wherein each of said four signals includes a predetermined packet format, said packet format including a packet field and an access control field and wherein said respective collision avoiding means further comprises: means for reading the contents of said access control field of said respective read signals and   means for overwriting the contents of said control field of any packet on said respective path.   
     
     
       9. The system defined in claim 7 wherein said respective collision avoiding means further comprises: means for writing first contents in a busy control field of said respective read signal, said first contents for indicating that said station has a packet to transmit.   
     
     
       10. The system defined in claim 9 wherein said respective collision avoiding means further comprises: means, responsive to second contents of said busy control field of said respective read signal, for writing a packet on said respective path, said second contents for indicating that said respective path is not busy.   
     
     
       11. The system defined in claim 9 wherein said respective collision avoiding means further comprises: means for reading the contents of a busy control field of said respective read signal, said read signal busy control field having been read prior to its being overwritten with said first contents,   means, responsive to said priorly read signal busy control field indicating that said respective path is not busy and responsive to the event that no station on said respective path was unable to transmit a packet, for writing an end cycle packet on said other respective path.   
     
     
       12. The system defined in claim 7 wherein said respective collision avoiding means further comprises: means, responsive to second contents of a busy control field of said respective read signal, for writing a packet on said respective path, said second contents for indicating that said respective path is not busy.   
     
     
       13. The system defined in claim 11 wherein said respective collision avoiding means further comprises: means, responsive to said end cycle packet on said other respective path, for initiating a start cycle packet on said respective path, said start cycle packet being transmitted by a head station on said respective path and said start cycle packet including a start cycle field responsive to which a station on said respective path may be initialized and permitted to transmit a packet.   
     
     
       14. Station apparatus adapted for use in a communications system, said station apparatus being adapted to be coupled to each of said signal paths and including means adapted to write a first signal on said first path and means adapted to read a second signal from said second path and characterized in that said signals on said first path include first control and data signals,   said signals on said second path include second control and data signals,   said first and second paths are, separate independent, unidirectional and oppositely directed with respect to each other, and   said station apparatus further comprises:   means for avoiding collisions on said first path, said first path collision avoiding means including   means for reading a third signal from said first control signals on said first path, said first path reading means being adapted to be coupled to said first path in a manner logically preceding said first path writing means,   means for avoiding collisions on said second communications path, said second path collision avoiding means including   means for writing a fourth signal as one of said second control signals on said second path, said second path writing means being adapted to be coupled to said second path in a manner logically succeeding said second path reading means.   
     
     
       15. The station apparatus defined in claim 14 wherein each of said four signals includes a predetermined packet format, said packet format including a packet field and an access control field and wherein said respective collision avoiding means further comprises: means for reading said access control field of said respective read signals and   means for overwriting said control field of any packet on said respective path.   
     
     
       16. The station apparatus defined in claim 14 wherein each of said respective collision avoiding means further comprises: means for writing first contents in a busy control field of said respective read signal, said first contents for indicating that said station has a packet to transmit.   
     
     
       17. The station apparatus defined in claim 16 wherein said respective collision avoiding means further comprises: means, responsive to second contents of said busy control field of said respective read signal, for writing a packet on said respective path, said second contents for indicating that said respective path is not busy.   
     
     
       18. The station apparatus defined in claim 16 wherein said respective collision avoiding means further comprises: means for reading a busy control field of said respective read signal, said read signal busy control field having been read prior to its being overwritten with said first contents,   means, responsive to said priorly read signal busy control field indicating that said respective path is not busy and responsive to the event that no station on said respective path was unable to transmit a packet, for writing an end cycle packet on said other respective path.   
     
     
       19. The station apparatus defined in claim 14 wherein each of said respective collision avoiding means further comprises: means, responsive to a busy contol field of said respective read signal indicating that said respective path is not busy, for writing a packet on said respective path.   
     
     
       20. The station apparatus defined in claim 18 further comprising said station being adapted to operate as a head station, and wherein said respective collision avoiding means further comprises: means, responsive to said end cycle packet on said other respective path, for initiating a start cycle packet on said respective path, said start cycle packet being transmittted by said head station on said respective path and said start cycle packet including a start cycle field responsive to which a station on said respective path may be initialized and permitted to transmit a packet.